1. Field of the Invention
The present invention relates to an Active Matrix Organic Light Emitting Diode (AMOLED) pixel, and more particularly, to a method for driving a current-driven AMOLED pixel.
2. Description of the Related Art
Accompanying with the progress of the information technology, more advance models for the variety of information devices, such as computer, mobile phone, and personal digital assistant (PDA), are continuously being developed. The display always plays a significant role in these information devices. Specifically, the flat panel displays are gradually becoming more popular since it provides the advantages of being thinner, lighter, and power saving.
Among the various flat panel displays, the Active Matrix Organic Light Emitting Diode (AMOLED) display comprises the advantages of wider view angle, better color contrast, lighter and thinner, faster response and lower cost. The Active Matrix Organic Light Emitting Diode (AMOLED) display is thus quite suitably applied in, for example, electronic clocks, mobile phones, PDAs, and digital cameras.
Referring to FIG. 1, FIG. 1 is a schematic view of a voltage-driven AMOLED pixel. As shown in the drawing, the AMOLED pixel comprises a switching TFT (Thin Film Transistor) 110, a driving TFT 120, a capacitor 130, and an Organic Light Emitting Diode (OLED) 140. The display gray level is determined by the voltage on the data line. When the scanning line is scanning the pixel, the switching TFT 110 is turned on to transmit the voltage on the data line to the gate of the driving TFT 120, so as to obtain a gate voltage Vg which is used to drive the required current to flow through the OLED 140 for displaying. The current Id flowing through the OLED 140 is generally expressed by following equation:Id=(1/2)k(Vgs−Vth)(Vgs−Vth)  (1)
However, since the threshold voltage Vth and the mobility of the driving TFT 120 for different pixels are different due to its uneven manufacturing processes, the current Id flowing through the OLED 140 is not the same under the condition of the same gray level voltage. Thus, the problem of an uneven display picture occurs.
Therefore, a current-driven AMOLED pixel is developed. Referring to FIG. 2, it is a schematic view of a conventional current-driven AMOLED pixel. As shown in the drawing, the AMOLED pixel comprises a first TFT 210, a second TFT 220, a third TFT 230, a capacitor 240, a driving TFT 250, and an OLED 260. When it is operating, at first, a scanning control signal of the scanning line is used to turn on the first TFT 210 and the second TFT 220, such that the current provided by the current source flows through the second TFT 220 to charge the capacitor 240. Meanwhile, the memorized gate voltage makes the current flowing through the first TFT 210 and the third TFT 230 are equal to the current of the current source. Then, when the scanning control signal of the scanning line is cut off (SCAN OFF), the gate voltage previously memorized is used to control the driving TFT 250, such that the current flowing through the OLED 260 is equal to the current of the current source so as to display the required brightness.
However, when this current-driven AMOLED pixel is used to display contiguous frames having a great variance in its display gray level, the incorrect picture display problem due to an insufficient discharge of the gate voltage memorized on the capacitor 240 may occur. For example, assuming that the required current of the current source for the nth frame is 20 μA, a voltage Vn is thereby stored on the capacitor 240. However, the required current of the current source for the n+1th frame is only 1 μA; thus, the voltage stored on the capacitor 240 has to discharge to Vn+1 within a very short period. The insufficient discharge problem thereby occurs.